Declare system property usage (for Gradle 7 configuration cache)

[C-Otto]

With Gradle 7 M3 and --configuration-cache I get an error that the CPD plugin reads a system property that is not declared.

org.gradle.api.InvalidUserCodeException: Read system property 'file.encoding'
	at org.gradle.configurationcache.SystemPropertyAccessListener.systemPropertyQueried(SystemPropertyAccessListener.kt:92)
	at org.gradle.internal.classpath.Instrumented.systemProperty(Instrumented.java:83)
	at org.gradle.internal.classpath.Instrumented.systemProperty(Instrumented.java:77)
	at de.aaschmid.gradle.plugins.cpd.CpdExtension.<init>(CpdExtension.java:26)
	at de.aaschmid.gradle.plugins.cpd.CpdExtension_Decorated.<init>(Unknown Source)

Possible fix: https://docs.gradle.org/7.0-milestone-3/userguide/configuration_cache.html#config_cache:requirements:undeclared_sys_prop_read

[aaschmid]

hm ... actually thought that this is already fixed. However it seems not to.
Thanks for reporting @C-Otto. I will try to fix it asap for staying compatible with the next version.

[aaschmid]

Duplicate of #54 and there I at least partly fixed it ;-)

However never released it as you did not confirm the fix...

[C-Otto]

Oh sorry, I forgot about the other issue... is there anything I can do now?

[aaschmid]

No, I thing I got the remaining problem about this and I think about fixing it will by using default encoding of UTF-8